Dimensioning and Tolerancing Handbook Episode 3 Part 4 potx

20-16 Chapter Twenty Once the machine stabilized (about 2 hours), the largest drift within any two hour segment in a single axis was approximately 0.4 µm, with individual spikes of 0.3 µm over a 30-minute time frame. Two additional 24-hour versions of this test were run with the same level of results. It is critical to note that the charts clearly display a direct correlation between temperature change and displacement, very close to a linear relationship. Tolerances on Tooling Components and Assemblies What needs to be kept in mind on this issue is that the “enhanced-accuracy” CMM was justified principally to measure critical features on tooling components and assemblies. In addition, we were clearly aware (up front) that this CMM (or any CMM) was not capable of measuring every feature we considered critical to process or function. For example, one of the restrictions on a contact CMM is probe diameter. The smallest “standard” probe tip available is 0.3 mm, which restricts measurements on an inside radii or diameter. A large percentage of the features of size have tolerances of 1.25 µm to 2.5 µm with feature location tolerances of 5 µm. I believe I would be conservative in saying that greater than 50% of the features that are measured on this CMM are < 5 µm. These are “current” tolerances defined on tooling drawings at this time. If we look back at one of the original “assumptions” (#1. 0.5 µm is accurate enough to tell us what effects the tool shapes have on the forming process), this was a “worst-case” statement which included accuracy and repeatability of the measurement system. What has been discussed so far has been only “repeatability.” Miscellaneous Feature-Based Measurement Tests It is essential that the results from the thermal drift test are understood to be based on a simple measurement within a small known envelope of 25 mm, so accuracy and repeatability are at their best. Where it starts becoming more difficult is in measuring other types of geometric features within a larger envelope, such as perpendicularity, cylindricity and profile, to name a few. It takes a significant number of points on a given feature to get an accurate representation of its geometry. A general rule to note is that as you increase the number of points, the better the accuracy and repeatability. There are exceptions, but in general this holds true. (6) Miscellaneous Variables Aid in Decreased Confidence of Measured Results In addition to temperature, there are many other variables that influence accuracy and repeat- ability. Some of these variables are humidity, contamination, types of probes due to stability (stiff- ness) such as the difference between steel shafts versus ceramic and carbide, probe speed, and fixturing. The list goes on and on. The key item at this time that is restricting our leap into the sub-micrometer capability we need (and have been striving for) is “temperature.” (7) Summary The “great” part about our CMM is that it is exceeding the specifications committed to by Brown & Sharpe/Leitz. They were aware from the beginning that our expectations of their system was to push it well beyond their stated capability. They also mentioned that tight temperature control would be necessary to accomplish this task. I sincerely feel the level of temperature control I’m stating here is also needed in many other measurement applications at our site to reduce current inaccuracies. I hope I have convinced the readers of this memo on the need for tight temperature controls to achieve sub-micrometer mea- surement capability on this type of measurement system. I will need approval for additional ex- penses of $35K to achieve the defined controls for the CMM room. If there are any questions, I would be happy to address them as best I can. END of MEMO. All funds were approved based on this presentation. Measurement Systems Analysis 20-17 20.3 CMM Performance Test Overview The testing was done on a Brown & Sharpe/Leitz PMM 654 Enhanced Accuracy CMM to determine the machine’s capability and the confidence with which various features could be measured. There are a variety of parameters affecting the repeatability of measuring a geometric element on a CMM. These parameters can be separated roughly into three categories: environmental, machine, and feature-dependent parameters. These include, but are not limited to, the following: 1) Environmental • Room (and part) temperature stability • Room humidity • Vibration • Dirt and dust in room • Airline temperature stability 2) Machine • Settling time (probing speed, probing offset, and machine speed) • Probing force (upper and lower force, trigger force, and divider speed) • Flexibility of probe setup (probe deflection) • Multiple probe tips (star probe setups and magazine changes) 3) Feature Dependent • Size (surface area) of feature • Number of points per feature • Surface roughness (form) of the part • Scanning speed The following three sections will add detail to the above three categories with insight to the testing completed. This should be considered summarized information that leads to the final development of the capability matrix — the final goal of “measurement methods analysis in a submicrometer regime.” The scope of these tests is intended to do whatever is necessary to have Six Sigma measurement capabilities for all geometric controls of interest, less than 1 µm. Many of the machine (Section 2) and feature-dependent (Section 3) tests have graphs showing a visual representation of the data. For convenience, these will not be referred to by graph number and will be located within the test section to allow better use of space. 20.3.1 Environmental Tests (Section 1) 20.3.1.1 Temperature Parameters To understand the relationship between the room environment and the CMM’s results a “thermal drift test” that tests for thermal variation error (TVE) was completed. This test is outlined in the ANSI/ASME Standard B89.1.12M and is called “Methods for Performance Evaluation of Coordinate Measuring Machines.” 20-18 Chapter Twenty To run this test, the CMM was parked in its home (upper, left, back corner) position for a period of six hours. This allows the machine enough time to stabilize if necessary. Then using five points, a 25-mm sphere was measured three times, reporting the average x, y, and z center position, diameter, and form. This measurement sequence was repeated for a minimum of 12 hours, and the results graphed opposite the temperature of the three axes scales. Temperature compensation was enabled at the beginning of every sequence. The range of the drift over the full length of the test was not the critical variable. Rather, it is the amount of drift that occurs over the length of time equivalent to the longest program used to measure a component or assembly. In this case, the interest was in the maximum time segment of two hours. TVE Test # 1: X Y Z Coordinate range (mm) 0.00417 0.00080 0.00068 Temperature range ( o C) 0.10040 0.08752 0.12872 This TVE test was run for a period of 56 hours in the new lab with temperature centered on 20 o C. The y and z axes showed an amazing linear response to the temperature of their respective axis. These test results prove that controlling the temperature of the machine axes is essential to the performance of the CMM. However, the results were not as good as expected and raised some new questions. First, why does the x-axis not respond to its temperature in a linear manner? Was there another parameter creating a greater effect on the x-axis than temperature? If so, what was that parameter? Also, why was the x-range so much larger than the y and z ranges? Finally, why do all three axes show a large decrease in temperature at the beginning of the measurement cycle? Was it the fact that the machine is running? (You would logically expect the machine to heat up, not to cool down when running.) Or was it the position of the machine when resetting in the home position versus its position when measuring the sphere? If so, what was causing the temperature drop? TVE Test # 2: X Y Z Coordinate range (mm) 0.00068 0.00053 0.00081 Temperature range ( o C) 0.04247 0.06178 0.10812 The next step was to run a shortened version (24 hours) of the same test to ensure the results of the first test were repeatable. When duplicating results, it is essential each step of the original test is followed exactly. The results were very similar to those from the first test. The y and z axes continued to have a strong linear relationship with their axes temperatures, while x was definitely nonlinear in nature. The initial decrease in all three axes temperatures was again evident in the first two hours of the test. In this test all three axes’ temperatures were also plotted against one another, showing that all three axes were following the same pattern. It was evident that whatever was creating the fluctuations in one axis was also affecting the other axes. When looking at the magnitude of the temperature drop, the z-axis had the largest tempera- ture range followed by the y and then the x axis. In addition, the three axes temperature plot revealed a great deal of stratification in the room (over a 0.3 o C difference) between the y and z axes and the x-axis. It is highly possible such a large amount of stratification could cause problems when attempting to hold the room environment constant. Finally, the y-axis temperature was displaying a cyclical pattern about 40-45 minutes in length. A closer inspection of the first test showed a similar pattern as well. This test left four questions to be answered: Measurement Systems Analysis 20-19 1) What machine or environmental parameter was causing all three axes to decrease in temperature at the beginning of every run? 2) Why was the x-axis displaying a nonlinear relationship to its axis temperature? Is there some other outside parameter affecting its performance? 3) Would the stratification of the room create any performance or room stability problems? If so, what was creating this stratification? 4) What was causing the cyclical effect observed in the y-axis? TVE Test # 3: X Y Z Coordinate range (mm) 0.00167 0.00072 0.00135 Temperature range ( o C) 0.04762 0.06693 0.11585 The next TVE test was designed to test whether the decrease in temperature occurred directly after the machine began to run. The temperatures of all three axes were recorded while the machine was resetting in its home position for six hours before measuring the sphere for 24 hours. The results of this test clearly indicated the machine reached a higher temperature plateau when placed in the home position. Either the movement of the machine or the machine placement was causing this change in temperature. Based on this, the decrease was being caused either by the room environment or the temperature of the air exiting the air bearings. At this point, a sensor was placed directly within the air line entering the room to monitor the temperature going into the air bearings. The results showed the temperature going into the air bearings was indeed higher than the room temperature. Could the air bearings be closer to the axes scales at certain positions of the machine? Or in the case of the z-axis, was the ram being warmed up due to the higher temperature air exiting from the air bearings? Questions arose regarding whether temperature compensation would create problems in the result- ing data if it were activated. An additional test was run without temperature compensation. Additionally, there was at least one rest period of six hours where the machine was left directly above the sphere. This data would tell us if the position of the machine was causing the temperature drop. Finally, these test results displayed the y and z axes were again linear to temperature while the x-axis was not. The temperature of the three axes continued to follow one another, and the same amount of stratification was evident. However, the cyclical pattern of the y-axis was not displayed in this test. TVE Test # 4: X Y Z Coordinate range (mm), (temp comp on) 0.00092 0.00051 0.00133 Coordinate range (mm), (temp comp off) 0.00092 0.00048 0.00113 Temperature range ( o C) 0.08336 0.09782 0.16476 In this test, the machine was placed in the home position for six hours, run for 12 hours, placed in the home position for six hours, run for 12 hours, placed directly above the sphere for six hours, and run for twelve hours. The sphere was measured with and without temperature compensation to see if any differ- ence did exist in the results. The results indicated the position of the machine was causing the change in temperature to occur. In all three axes, there was a definite rise in temperature when the machine was in the home position. When 20-20 Chapter Twenty the machine was left to rest above the sphere, however, no similar rise in temperature was evident. Additionally, the test showed only a simple bias between the data taken with and without temperature compensation. The data collected up to this point was indeed valid. Finally, the cyclical effect that had disappeared in the previous test had resurfaced not only in the y-axis but also in the z-axis. Based on this data, a new approach was taken to control the room environment (based on the memo shown at the beginning of section 1). A new air-flow system was added to ensure a uniform air flow moving over and away from the CMM. This would prevent warm pockets of air from being trapped around the machine. Test # 4 was replicated. TVE Test # 5: X Y Z Coordinate range (mm), (temp comp on) 0.00047 0.00042 0.00052 Coordinate range (mm), (temp comp off) 0.00052 0.00047 0.00051 Temperature range ( o C) 0.03928 0.04332 0.04111 Based on these results, test #5 was replicated two more times to ensure a high degree of confidence in the measured results. TVE Test # 6: X Y Z Coordinate range (mm), (temp comp on) 0.00042 0.00038 0.00049 Coordinate range (mm), (temp comp off) 0.00048 0.00046 0.00050 Temperature range ( o C) 0.04211 0.04182 0.04132 TVE Test # 7: X Y Z Coordinate range (mm), (temp comp on) 0.00045 0.00040 0.00050 Coordinate range (mm), (temp comp off) 0.00050 0.00042 0.00054 Temperature range ( o C) 0.03723 0.04123 0.03998 It is interesting to note that the cyclical effects stayed present in the last three tests, but to a lesser degree. Further temperature optimization was not pursued due to current satisfaction in the noted results. 20.3.1.2 Other Environmental Parameters There are obviously more environmental parameters than simply temperature. Humidity, vibration, dirt and compressed air quality are generally considered less important, but were determined to be well within specifications. The pressure and temperature of the compressed air was also within specifications before the ma- chine was installed. However, due to concerns arising from the TVE tests, the compressed air was exam- ined again. Sufficient pressure was being supplied to the machine and the temperature (although higher than room temperature) was within specification. Finally, the dust content of the room was lowered slightly by adding floor mats in the buffer room and by sealing off miscellaneous areas. Based on the Six Sigma capabilities being driven for in the submicrometer regime, it is essential the room environment be as stable as possible. Uniform air flow and temperature over the CMM must be constant, as any change will be recognized. Measurement Systems Analysis 20-21 20.3.2 Machine Tests (Section 2) 20.3.2.1 Probe Settling Time The Leitz PMM 654 machine was installed so the factory default machine parameters were active. These default settings have been optimized for maximum accuracy and throughput when using the machine for a majority of the applications. However, these settings can be changed to improve accuracy or throughput on out of the ordinary applications. For example, the force applied by the probe head must be lowered in order to measure a thin, flexible part. The machine settings marked as important to test are the probe settling time and probe force. Machine Test #1: Z-Axis single-point measurement versus probe settling time (see Fig. 20-1) The probe settling time is a function of two probe settings: the probing speed (mm/sec) and the probing offset (mm). By decreasing the probing speed and increasing the probing offset (thereby increas- ing settling time), we should see an increase in the performance of the machine. To test this theory, a single point in the z-axis was measured 25 times and its Six Sigma repeatability was calculated. This sequence was repeated using various combinations of the two settings. The results displayed unique changes in the repeatability of single-point measurement as the settling time increased from 0.125 to 1 second. These results were contradictory to the original hypothesis that increasing the settling time would increase machine performance. Figure 20-1 Z-Axis single-point repeatability 0 1 2 3 4 5 6 7 8 9 10 1 0.5 0.25 0.125 Settling Time (sec) 6 sigma (10^-4 mm) 0.5 mm Prb Offset 2.0 mm/sec Touch Sp 20-22 Chapter Twenty Figure 20-2a Form Six Sigma versus probe settling time (10-mm sphere) Figure 20-2b Sphere form versus probe settling time (25-mm sphere) Machine Test #2: Sphere form versus probe settling time (see Figs. 20-2a and 20-2b) In this test, three different probes were calibrated on a 10-mm sphere. This same sphere was then remeasured 25 times using a 29-point pattern, reporting the sphere’s mean form and Six Sigma value. The 0 2 4 6 8 10 12 14 16 18 20 22 24 2 0.2 Probe Touch Speed (mm/sec) 6 igma Sphere Form (10^-4 mm) 5.0 mm Probe 3.0 mm Probe 1.0 mm Probe 0 2 4 6 8 10 12 14 16 2 0.2 Probe Touch Speed (mm/sec) 6 sigma Sphere Form 6 sigma (10^-4 mm) 1 mm probe 3 mm probe 5 mm probe Measurement Systems Analysis 20-23 Figure 20-3 Probe speed versus sphere form first series of measurements were taken using the default probe speed of 2 mm/sec. A second series of measurements were taken at 0.2 mm/sec (the probe was recalibrated at the lower speed before measure- ment). This entire procedure was then repeated with a 10-mm sphere. The results show a slight improvement in the mean form when lowering the probe speed. These results were similar to those from the single-point repeatability. This is more than likely due to the design of the Leitz probe head, where the actual probe point is registered as the head is pulling away from the part. Therefore, the small range of this test had a limited effect on the machine’s performance, which is adequate based on the speculated range of operation. Machine Test #3: Probe speed versus sphere form (see Fig. 20-3) This test was run to get a better idea of the machine’s response over a greater range of settling times. Using the default probe offset of 0.5 mm, the following probe speeds (mm/sec) were tested: 4, 2, 1, 0.5, 0.25, 0.125, and 0.0625 At each probe speed, two different probes were calibrated on the 25-mm sphere. This sphere was then remeasured using a 29-point pattern, with the form, diameter, and probe deflection being reported. The results again showed limited decrease in the sphere form as the probe speed decreased, regardless of which probe was tested. At this time, there is no evidence to support the idea that decreasing the probe settling time will increase the performance of the machine. Within the range of values tested, there was no evidence of relationship between settling time and machine performance. 5 6 7 8 9 10 6 sigma Sphere Form (10^-3 mm) 1.5 mm Probe 3.0 mm Probe 0 1 2 3 4 0 0.5 1 1.5 2 2.5 3 3.5 4 Probe Speed (mm/sec ) 20-24 Chapter Twenty 20.3.2.2 Probe Deflection The more flexible the probe shaft becomes, the more difficult it becomes to measure in an accurate and repeatable manner. To compensate for this problem, the Leitz probe head creates a deflection matrix, which attempts to map out the amount and direction the probe shaft will deflect. The following is a layout of this matrix: xx xy xz yz yy yz zx zy zz Figure 20-4 Sphere form versus probe trigger force (10-mm sphere) 0 2 4 6 8 10 12 14 16 18 20 22 24 0.5 0.05 Probe Trigger Force (N) 6 sigma Sphere Form (10^-4 mm) 5.0 mm Probe 3.0 mm Probe 1.0 mm Probe Machine Test #4: Sphere form versus probe trigger force (see Fig. 20-4) Another assumption made before testing began was that lowering the probe head “trigger force” would improve the machine’s performance. By varying the probe force, it should be possible to decrease the deflection to which the probe shaft is subjected. This theory was put to the test using three different probe tips calibrated on the 25-mm sphere. This sphere was then remeasured 10 times using a 29-point pattern, reporting the mean form and Six Sigma value. The first series of measurements were taken using the default trigger force of 0.5 N. A second series of measurements were taken using 0.05 N trigger force (the probe was recalibrated at the lower trigger force before measurement). This entire procedure was then repeated using the 10-mm sphere. The results show an inconsistent relationship between the probe force and sphere form. It was determined that probe force is really a function of several machine settings; upper and lower force, trigger force, and divider speed. Further testing showed that it was possible to influence the form and diameter of the measured sphere by changing these parameters. [...]... sigma (10^ -4 mm) 8 7 6 5 4 3 2 1 0 0 0.1 0.2 0 .3 0 .4 0.5 0.6 0.7 Hole Diameter (mm) Circle Dm X Position Y Position Roundness Figure 20-8 Circle features versus hole diameter 10 6 sigma (10^ -4 mm) 9 8 7 6 Figure 20-8 Circle features versus hole diameter 5 4 3 2 1 0 6 18 30 44 Ring Gage Diameter (mm) Cylinder Diameter Cylinder Position Figure 20-9 Cylinder features versus hole diameter 56 20-29 20 -30 Chapter... Caskey.19 93 Measurement Uncertainty Considerations for Coordinate Measuring Machines MISTR 5170, Precision Engineering Division, NIST: April 19 93 The American Society of Mechanical Engineers 1995 ASME Y 14. 5M-19 94, Dimensioning and Tolerancing New York, New York: The American Society of Mechanical Engineers The American Society of Mechanical Engineers 1995 ASME Y 14. 5.1M-19 94, Dimensioning and Tolerancing. .. dominating parameter overshadowing any effect the axis may have had on the results 7 6 sigma (10^ -4 mm) 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Parallelism = 1-2, Angularity = 3- 6, Width = 7-10, Flatness = 11-15 Single Probe Star Probe Figure 20-15 25-mm cube test—single versus star probe setup 20 .3. 3 .3 Contact Scanning Due to its unique probe head, the Leitz PMM can carry out constant contact... hole, on the other hand, is a range of values; therefore, its repeatability deteriorates as the number of points increase Measurement Systems Analysis 10 6 sigma (10^ -4 mm) 9 8 7 6 5 4 3 2 1 0 100 0 200 40 0 30 0 Gage Bar Length (mm) Min Length Max Length Bi-dir Probing Figure 20-10b Bidirectional probing versus varying lengths (y-axis) 10 9 6 sigma (10^ -4 mm) 8 7 6 5 4 3 2 1 0 0 10 20 30 40 50 60 No Points/Feature... Chapter Twenty Feature Based Test #3: Bidirectional probing versus varying lengths (x and y axis) (see Figs 20-10a and 20-10b) Six gage bars of lengths 25, 50, 100, 200, 250, and 40 0 mm were placed in the x- and y-axes The two end planes were measured using 32 points each, recording the minimum and maximum length of the bars In addition, a single point was taken on each end, and the bidirectional probing... 20 -36 Chapter Twenty Figure 20-17 Leitz PPM 6 54 capability matrix Measurement Systems Analysis Figure 20-17 continued Leitz PPM 6 54 capability matrix 20 -37 20 -38 Chapter Twenty Some of the NIST-traceable artifacts used for determining system accuracy and repeatability, and the types of features checked are listed below • 45 0-mm Moore bar (step gage used to determine linear displacement “X, Y, and. .. have not been taken “fully” into consideration and will spur tremendous development efforts for many years to come Measurement Systems Analysis 20.5 1 2 3 4 5 6 20 -39 References Hetland, Gregory A 1995 Tolerancing Optimization Strategies and Methods Analysis in a Sub-Micrometer Regime Ph.D dissertation Majlak, Michael L 19 94 Error Budgets Used in CMM Design and Application Studies Paper presented at ASPE... section to 32 points per section This entire procedure was then repeated 25 times If there were any temperature stability problems, their effects would be the same for all point density runs Measurement Systems Analysis 20 -33 16 6 sigma (10^ -4 mm) 14 12 10 8 6 4 2 0 2 4 6 8 10 12 14 16 18 No of Points/section Cylinder Form Cylinder Dm Perpendicularity Straightness Cylinder Position Figure 20- 13 Cylinder... measurements, no further testing on scanning was done at this time 24 22 6 sigma (10^ -4 mm) 20 18 16 14 12 10 8 6 4 2 0 0 0.5 1 1.5 2 Scanning Speed (mm/sec) Circle Dm X Position Y Position Roundness Figure 20-16 Circle features versus scanning speed 20 .3. 3 .4 Surface Roughness It is generally accepted that the surface roughness of a part/ feature will affect the repeatability of a single point being... gage and repeatability of the features Additionally, neither the x or y axis seemed to perform better than the other These tests have been limited to the 25 mm × 25 mm area on the ends of the gage blocks 10 6 sigma (10^ -4 mm) 9 8 7 6 5 4 3 2 1 0 0 100 200 30 0 40 0 Gage Bar Length (mm) Min Length Max Length Bi-dir Probing Figure 20-10a Bidirectional probing versus varying lengths (x-axis) 20 .3. 3.1 Number . settling time and machine performance. 5 6 7 8 9 10 6 sigma Sphere Form (10^ -3 mm) 1.5 mm Probe 3. 0 mm Probe 0 1 2 3 4 0 0.5 1 1.5 2 2.5 3 3.5 4 Probe Speed (mm/sec ) 20- 24 Chapter Twenty 20 .3. 2.2 Probe. hole diameter 0 1 2 3 4 5 6 7 8 9 10 0 0.1 0.2 0 .3 0 .4 0.5 0.6 0.7 Hole Diameter (mm) 6 sigma (10^ -4 mm) Circle Dm X Position Y Position Roundness 0 1 2 3 4 5 6 7 8 9 10 6 18 30 44 56 Ring Gage Diameter. range (mm), (temp comp on) 0.00 042 0.00 038 0.00 049 Coordinate range (mm), (temp comp off) 0.00 048 0.00 046 0.00050 Temperature range ( o C) 0. 042 11 0. 041 82 0. 04 132 TVE Test # 7: X Y Z Coordinate